1. Field of the Invention
The present invention relates to an achromatic lens system which is used in objective lens elements of an astronomical telescope, in collimator lens elements, and the like.
2. Description of the Related Art
An achromatic lens system in which a positive lens element is provided between two negative lens elements has been well known. Further, in order to decrease the secondary spectrum of axial chromatic aberration, an achromatic lens system which employs a positive lens element made of fluorite has been widely used. However, fluorite is known as a material which is difficult to be machined, and is expensive. Furthermore, recent CCD cameras have sensitivity in the near-infrared range, while achromatic lens systems which use fluorite cause the over-correcting in the near-infrared range. As a result, suitable chromatic correction in the visible range and the near-infrared range cannot be simultaneously performed.
For example, in the prior art, when astronomical photography is performed with a CCD camera, an infrared-cut filter has to be used in order to cut light in the near-infrared range because chromatic correction by an objective lens system (achromatic lens system) of the CCD camera is insufficient in the near-infrared range though the CCD camera has sensitivity in the near-infrared range. If sensitivity of CCD cameras in the near-infrared range is utilized, clear celestial images can be photographed through image processing technologies. The same can be applied to surveillance cameras which utilize near-infrared light.